A heartbeat to be measured in order to confirm the health of a human is expected not only to serve as a health monitor, but also to be applied to assessment of stress, a predictive detection of hypnagogic state, when driving a vehicle, or the like. More specifically, it is recognized that, since a rhythm adjustment of the heartbeat may be made by an autonomic nerve, an analysis of change in heartbeat interval permits to estimate an influence of the autonomic nerve on the heartbeat, and a mental stability and a degree of stress, which may cause such a change.
For example, a condition, which forces an operator or a driver to take a seat for a long period of time along with an operation of a computer or a driving of a vehicle for a long period of time, or a long-distance movement, may give the operator or driver a mental load (stress). There have conventionally been problems that such a mental load may sometime cause various kinds of stress-related illness. It has been significant to properly evaluate the stress applied to the human body to avoid such a illness in advance, and there has been proposed an approach to evaluate the stress through analysis of heartbeat variability.
A way to attach electrodes to a body of a subject to obtain an electrocardiogram has conventionally generally been taken to measure the heartbeat. The heartbeat variability may be calculated by reading out peak intervals (RRI) of R-waves appearing in a shape of spike in the cardiac electrogram based on a threshold value as set. A frequency analysis of such a heartbeat variability permits to provide a stress evaluation index.
More specifically, a frequency analysis of time variation of the heartbeat results in the fact that the spectrum peak appears in each of the low frequency component having the bandwidth of from about 0.03 Hz to about 0.15 Hz and the high frequency component having the bandwidth of from 0.15 Hz to about 0.45 Hz. The respective peak values will be referred to as “LF” and “HF”, respectively. LF represents a state of activity of both of the sympathetic nerve and the parasympathetic nerve, and HF represents a state of activity of only the parasympathetic nerve. Here, in view of the facts that the sympathetic nerve predominates in a state in which a human body is subject to stress, and the parasympathetic nerve conversely predominates in a state of relaxation, a value of LF/HF may be deemed as an index of activity of the sympathetic nerve, namely as a stress evaluation value. When values of integral of frequency spectrums of from 0.03 to 0.15 Hz and from 0.15 to 0.45 Hz are referred to as “LF” and “HF”, respectively, a value of LF/HF may be deemed as an index.
Concerning an example of the conventional method to make a stress evaluation based on variation of heartbeat interval in this manner, JP 2001-95769 A, JP 2005-218595 A and JP 2008-99876 A disclose such a method.
JP 2008-253538 A discloses an example of a method in which a heartbeat is measured in a non-contact state with the use of a microwave and then a stress evaluation is made, in a different manner from the method utilizing the electrocardiograph. This publication describes a method comprising the steps of irradiating radio wave to a subject, receiving a reflected wave from the subject, converting a signal as received into a digital signal, calculating a peak interval (a PP interval) of the signal, and analyzing a heart rate variability (HRV) index from the PP interval based on an approach such as a Maximum Entropy Method to provide a low frequency component and a high frequency component as separated of the frequency region.